Polyethylene, during processing operations such as extrusion, is known to experience a phenomenon described as melt fracture in which, upon exiting the extruder die, the extrudate has a highly irregular surface. The irregular surface is rough, and it does not have a consistent surface for fabricating a quality finished article or for producing an aesthetically pleasing article. Linear polyethylenes such as LLDPE (linear low density polyethylene) and VLDPE (very low density polyethylene), due to an inherent molecular structure/melt rheology characteristic, are highly susceptible to melt fracture while highly branched polyethylene such as LDPE (low density polyethylene) is significantly less prone to melt fracture. With high molecular weight (low melt index), narrow molecular weight distribution, narrow (uniform) comonomer distribution linear polyethylene, the melt fracture phenomenon is especially severe under relatively low temperature extrusion conditions such as temperatures below 160 degrees C., ranging as low as about 100 degrees C.
Conventional techniques for the elimination of melt fracture are to raise the process temperature thus reducing the polymer's viscosity, which results in a corresponding lower shear strain at the die; to decrease the output rate thus decreasing the shear rate and corresponding shear strain at the die; or to increase the die shear rate thus increasing the polymer's viscous energy generation to raise the localized melt temperature for an effect similar to raising the process temperature. These techniques reduce the viscosity of the polymer and the resulting melt fracture. However, there are deficiencies in these techniques that make them unacceptable for processing materials under low temperature processing conditions, i.e., temperatures below 160 degrees C.
The requirement for processing temperatures of less than 160 degrees C. is desirable when extruding a resin formulation containing an organic peroxide, a thermally sensitive additive. When extruding a polyethylene formulation containing an organic peroxide, raising the process temperature is not a desired option to eliminate melt fracture in view of the problem of scorch, i.e., premature crosslinking caused by the decomposition of the organic peroxide. Decreasing the output rate is also not desirable because it increases the cost of manufacturing the extrudate product. Finally, increasing the die shear rate is similar in effect to raising the melt temperature with the attendant scorch.
Another approach to addressing the melt fracture phenomenon is to redesign the process equipment; however, this involves designing equipment for a specific molecular weight resin which limits the usefulness of the equipment, and, of course, raises the cost. Incorporating processing aid additives is another approach, but this is expensive and may affect other product properties. Blending a lower molecular weight polymer with a relatively high molecular weight polymer has the disadvantage that it typically results in a product with properties inferior to the high molecular weight product's properties.
Co-extrusion methods have also been employed to overcome melt fracture, but these methods have been generally applied to tubular blown film processes rather than extrusion around wire or glass fibers, for example. And these blown film processes have been conducted at temperatures considerably higher than 160 degrees C. with resin formulations, which do not include peroxides. Further, the polyethylene resins used in these blown film processes have been of the heterogeneous type, which are not as susceptible to the melt fracture phenomenon.